'A third element has crept into this conflict, possibly from Iraq, Lebanon and Jordan, and its agenda has nothing to do with the Arab spring or the clamour for democracy.' Illustration by Toby Morison.

The two suicide car bombs in Damascus on 10 May were an alarming development. Before last December suicide bombs were unheard of in Syria. Now there have been 10 such attacks, becoming increasingly deadly – 55 died in the latest atrocity; and on 11 May another attack was thwarted in Aleppo, Syria's largest city, where a suicide bomber in a carwash killed five on 5 May.

Damascus and Aleppo are home to Syria's business and professional classes, who have not, in general, participated in the uprising, tending to remain loyal to the Assad regime. The suicide bombs have targeted government buildings, the security services and the ruling Ba'ath party's headquarters. While many civilians died in Thursday's blasts, significant numbers of security personnel have also been killed.

None of this suggests that the regime is carrying out these atrocities, as the opposition has claimed, although it is true that Syria has armed and backed extremist groups such as the Abu Nidal organisation and Hezbollah. Moreover, it is unlikely that the Free Syrian Army, the armed wing of the opposition, has appropriated methods that are the hallmark of jihadist, not secular, groups.

My fear is that a third element has crept into this conflict, possibly from Iraq, Lebanon and Jordan, and that its agenda has nothing to do with the Arab spring or the clamour for democracy.

On Friday an Islamist group calling itself al-Nusra (Victory) Front released a video in which it added the Damascus bombs to others it has already claimed. In the manner of al-Qaida-associated groups, al-Nusra has already established a high production value news outlet, called the Al-Manarah Al-Bayda Media Foundation. The spokesman highlighted the sectarian intentions of the attack, stating that Sunni Muslims need "protection" from the ruling Shia Alawites, who will be made to "pay the price".

And this apparently sinister development has not occurred in isolation. Saudi Arabia, Qatar and other Gulf states were keen to arm the Syrian revolution – not because they are lovers of democracy and reform, but because they would like to see Assad removed from power. Under pressure from the Islamist establishment inside their own countries (which has its own sectarian agenda), the Saudis in particular are also mindful of the 2005 assassination of Rafik Hariri (a close friend of the al-Sauds who held joint Saudi-Lebanese citizenship), which led to a long-standing personal vendetta between the Saudis and the Assad regime, which they held responsible.

The problem is that the Free Syrian Army, even well equipped, is unlikely to prevail, unaided, against Syria's mighty and highly professional armed forces, which in general remain loyal. Meanwhile, the advent of suicide bombers has given Assad a pretext for escalating the violence against his own citizens.

Given the course of events in Libya, the opposition were, understandably, gambling on foreign intervention, but none has been forthcoming: first, because long-term Syrian allies Russia and China stood by Assad and vetoed all relevant UN security council resolutions; second, because the US is reluctant to become embroiled in yet another costly – and dangerous – conflict, given its nine-year occupation of Iraq and its ongoing, decade-long, war in Afghanistan. Any American intervention is likely only after the presidential elections in November, and would be linked to an attack on Iran, which remains the main focus in the region.

Any hopes that Kofi Annan's peace initiative might succeed have been blown out of the water by the apparent arrival of an extremist group, or groups, intent on escalating the sectarian aspect of the conflict, which neither the regime nor the opposition can hope to control.

If the extremist groups manage to hasten the fall of the regime, their agenda is unlikely to end there. In post-Saddam Iraq, Abu Musab al-Zarqawi's al-Qaida offshoot fanned the flames of a Sunni-Shia sectarian war that was only extinguished by the US army's "surge" and General Petraeus's "Awakening" campaign, which overwhelmed the jihadis temporarily. But in Syria there are no US forces, no Petraeus in sight.

Whoever governs post-revolutionary Syria is unlikely to rule over a united country, but rather sectarian or ethnic pockets, engaged in ongoing battles with each other. The historical precedent here is Lebanon, which was mired in civil war from 1975 for 16 years.

Nor is the prospect of sectarian conflict confined to Syria's borders. Regional polarisation might see a Sunni bloc, headed by Turkey and Saudi Arabia and incorporating any number of extremist groups, facing off a Shia alliance led by Iran. Here we have an even more chilling template – from 1514 the Sunni Muslims of the Ottoman empire and the Shia Safavid of Persia battled over the region for more than a century, fuelled by their religious differences.